The primary cause of human outbreaks is from skin contact with contaminated hodes, leather, and animals. Natural infection from inhalation or ingestion is rare. The Canadian Center for Disease Control reports that the infectious dose in humans is 1300 organisms through inhalation. This equates to one-billionth of a gram (about the size of a speck of dust), which would be lethal to a single person. The U.S. military reports that the lethal dose is 8,000-10,000 spores. Once infected, the incubation period for anthrax is 7 days and, on average, symptoms begin within 2-5 days. When inhaled at the proper dose, the disease runs a swift sequence of events, and death can occur within 24-48 h. Spore size is also an important factor in determining their effectiveness. Particles from 2 to 5 µm would tend to be filtered out by the upper airway. It is expected that heavy smokers might be more susceptible to large particles.